In recent years, organic EL displays using electroluminescence (EL) of an organic material have been attracting attention as one of the next generation flat-panel displays replacing liquid crystal displays. Unlike voltage-operating liquid crystal displays, the organic EL displays are current-operating device, and there is an urgent need for development of thin-film transistors (thin-film semiconductor device) having excellent on-off characteristics as the driving circuits of the active-matrix display device.
As a thin-film transistor achieving excellent on-characteristics, a thin-film transistor having one semiconductor layer on the gate insulating layer, which is used as a projecting channel layer has been disclosed (patent literature 1). According to this technology disclosed, the thickness from the bottom surface of the projecting shape to the upper surface on each side is smaller than the thickness from the bottom surface of the projecting shape to the upper surface at the center of the projecting shape. More specifically, at the lower part of the projecting shape in the channel layer which is a current path, when current flows between the source electrode and the drain electrode through the lower part of the sides of the projecting shape in the channel layer, the thickness on the lower part on both sides of the projecting shape of the channel layer is smaller than the upper part of the projecting shape. Accordingly, it is possible to reduce the resistance component in the vertical direction of the channel layer. Accordingly, it is possible to achieve low resistance across the lower part of the projecting shape in the channel layer, and thus the on-state current in the thin-film transistor is increased. Note that, while the mobility of the non-crystalline silicon is approximately 1 cm2/Vs, the mobility of the crystalline silicon is large, and is approximately 100 cm2/Vs. Accordingly, in order to implement a thin-film transistor having excellent on-characteristics, the semiconductor layer is formed of crystalline silicon, for example.
As a thin-film transistor achieving excellent on-characteristics and off-characteristics, the a thin-film transistor including a crystalline silicon layer formed on the gate insulating layer, and a non-crystalline silicon layer formed on both sides of the crystalline silicon layer is disclosed (the patent literature 2). According to this technology, the non-crystalline silicon is irradiated with a laser beam from an opening between the source electrode and the drain electrode so as to crystallize the non-crystalline silicon as the channel layer. With this, the non-crystalline silicon in the central region of the channel layer is crystallized into crystalline silicon, and the crystalline silicon is formed as a path for current in the channel layer. Accordingly, the on-state current in the thin-film transistor increases. In addition, when the turn-off operation is performed in the thin-film transistor, a depletion layer formed in a reverse-bias state between the channel layer and the drain serves as a barrier for carriers, achieving the turn-off operation. However, in the depletion layer, electrons and holes are thermally formed, which results in thermal generation current that deteriorates off-characteristics. Furthermore, a gate voltage large on the turn-off side is applied; the depletion layer is formed in the channel layer on the side facing the gate electrode. Here, since a large electric field is applied in a concentrated matter in the depletion layer, the band in the channel layer is significantly curved. As a result, tunnel current is generated, which deteriorates the off-characteristics as the leakage current. According to this technique disclosed, a non-crystalline silicon layer is formed on both sides of the crystalline silicon layer. The band gap of the non-crystalline silicon is larger than the crystalline silicon. Accordingly, a large energy is necessary for thermally generating electrons and holes, and a potential barrier causing the tunnel effect is large as well. Accordingly, forming the non-crystalline silicon layer on both sides of the crystalline silicon layer prevents the generation of the thermal generation current and the tunnel leakage current, reducing the off-state current. As described above, forming a channel layer including crystalline silicon increasing the on-state current and non-crystalline silicon reducing the off-state current achieves excellent on-characteristics and off-characteristics.